Telephone network interface circuits (i.e., data access arrangements (DAAs)) for international applications require high voltage isolation and compliance with various country-specific parameters. The technical specifications for network connections vary widely in different countries and are intended for the protection of the network from harms caused by connections of terminal equipment thereto. Accordingly, telephone network interface applications require DAA-type solutions capable of high voltage isolation with flexibility to accommodate stricter and more varied performance requirements.
There are many considerations that must be taken into account when connecting terminal equipment to the telephone network. For example, hazardous voltages and currents placed on the telephone lines by customer equipment can damage central office equipment or injure personnel. Additionally, signal levels must be maintained within certain maximum limits in order to prevent overloading and cross talk. Compliance with specified on-hook impedances is also required because the central office must evaluate line conditions, and the on-hook impedances must also be sufficient to drive the telephone ringer. Balance of the line, referred to as impedance matching, is also important because impedance mismatch will produce hum and cross talk. Finally, certain time delays and the absence of signals on certain prescribed signaling frequencies are required in order to address the concerns of local operating companies about billing protection.
FIG. 1 depicts a prior art circuit 100 for interfacing end user terminal equipment to the telephone network. A load impedance Z.sub.L represents the specific impedance parameters of the telephone network. High voltage isolated switches 101, 102, and 103 are connected across and are programmed in combination with a transmission impedance Z.sub.S to provide the required impedance match to Z.sub.L. A disadvantage of using prior art circuit 100 in international markets is that a parallel array of a large number of high voltage isolated switches is needed to generate a reasonable image match (i.e., impedance match between a user side and a line side of the network) to meet the needs of several countries, each having their own country-specific parameters, such as return loss requirements. These high voltage isolated switches are expensive, and, if present in sufficient numbers, will make a network interface circuit too large to be practical where cost, size, and programmability are paramount concerns. Accordingly, there is a need to provide a simplified network interface that eliminates the extra high voltage isolated switches required for programming the network specific parameters.
FIG. 2 shows another prior art circuit 200 which is intended to produce an output impedance Z.sub.O to match a line side impedance Z.sub.N of a network. Circuit 200 includes a transmit path having an operational amplifier circuit A.sub.201, and a feedback path that includes an operational amplifier circuit A.sub.202 and an emulation impedance Z.sub.EM. The operational amplifier circuit A.sub.201 provides a gain of a voltage signal applied from signal source 210. Using standard circuit analysis techniques, Z.sub.O of circuit 200 is defined as: ##EQU1##
A proper selection of Z.sub.EM in circuit 200 is intended to set Z.sub.O to match the impedance of Z.sub.N. However, as one skilled in the art will realize from equation 1, Z.sub.EM is not easily separable from the other scaling terms (i.e., the other circuit elements (e.g., resistors) which scale Z.sub.EM). More specifically, Z.sub.O cannot be isolated into an impedance term (i.e., Z.sub.EM) and a separately distinguishable scale term. As such, circuit 200 suffers the disadvantage of having poor control of the output impedance Z.sub.O, because Z.sub.EM cannot be effectively scaled in a practical manner. Consequently, while circuit 200 is intended to provide an impedance match to Z.sub.N, this objective is frustrated by the difficulty in scaling Z.sub.EM to set Z.sub.O. Moreover, circuit 200 also suffers the disadvantage of not having voltage isolation between the line side and the low voltage side to protect against hazardous voltages and currents. Therefore, circuit 200 does not provide a practical solution for impedance emulation across a high voltage boundary.
Although other efforts have been made to address impedance emulation in a network interface context using amplifier-based circuits, these efforts have not been successful for several reasons. Among these reasons is that the prior art circuit topologies have not solved the noise problems associated with frequency-related amplifier effects. Accordingly, there is still a critical need for a simplified and less costly network interface that provides both impedance matching and voltage isolation between the line side and low voltage side of a network without using expensive high voltage isolation switches and the like.